1. Field of the Invention
The present invention relates to a pop noise suppression circuit for a semiconductor IC audio amplifier.
2. Description of the Related Art
FIG. 2 shows IC audio amplifier 50, negative feedback capacitor C53, and ripple removing capacitor C54. Capacitors C53 and C54 are externally connected to negative feedback terminal 51 and ripple terminal 52 of amplifier 50, respectively. In this figure, reference symbols R1 and R2 denote bias resistors for voltage-dividing a Vcc power supply voltage, thereby to determine a bias voltage; and R3, a resistor which, together with ripple removing capacitor C54, forms ripple filter "Frip" for removing ripple components of the bias voltage. Reference numeral 55 denotes a current source circuit which receives the output voltage from ripple filter "Frip" and supplies a current corresponding to the input voltage. Circuit 55 comprises bipolar transistors Q1 to Q7, and resistors R4 to R7, and R20. Reference symbols Q8 and Q9 denote transistors for a differential amplifier. The emitters of transistors Q8 and Q9 are commonly connected, and are also connected to the base of current source transistor Q6. Reference numeral 56 denotes an input terminal connected to the base of transistor Q8. The base of transistor Q8 is connected to the output terminal of ripple filter "Frip" via resistor R8. Reference numeral 57 denotes a load circuit formed of the differential transistor pair. Circuit 57 comprises current-mirror-connected transistors Q10 and Q11, and resistors R9 and R10. Reference symbol Q12 denotes an emitter-grounded transistor which receives the output from transistor Q8. Capacitor 58 is connected between the collector and base of transistor Q12. The collector of transistor Q12 is connected to current source transistor Q7. Reference symbol Q13 denotes an emitter-follower transistor which receives the collector output from emitter-grounded transistor Q12. The emitter of transistor Q13 is connected to ground terminal 59 via resistor R11, and is connected directly to output terminal 60. Negative feedback resistor R12 is connected between output terminal 60 and the base of transistor Q9, and resistor R13 is connected between the base of transistor Q9 and negative feedback capacitor C53.
Ripple filter "Frip" prevents the ripple components of the Vcc power supply from leaking to bias output terminal 52, via bias resistor R1, and the filter time constant thereof serves to reduce variations in the voltage passing along output line (ripple line) 61 of ripple filter "Frip" upon ON/OFF operation of the power supply voltage, thus suppressing generation of pop noise signals.
When the output voltage from output terminal 60 rapidly changes upon ON/OFF of the power supply, a current suddenly flows through a loudspeaker (not shown) connected to output terminal 60, this causes pop noise to be generated. Since pop noise can cause serious physical discomfort, it must therefore be suppressed. In order to do this, abrupt variations in the output voltage of output terminal 60 must be suppressed. In a conventional amplifier, such suppression is realized, using a time constant of ripple filter "Frip", such that a voltage on ripple line 61 is slowly varied upon ON/OFF of the power supply.
With this method, if the ON/OFF operation of the power supply is performed with a certain time interval, the pop noise suppression effect by ripple filter "Frip" can be obtained. However, when the ON/OFF operation is continuously and quickly repeated, in particular, when the ON operation is performed immediately after the OFF operation, pop noise is generated.
More specifically, since ripple removing capacitor C54 and negative feedback capacitor C53 have large capacitances, charges remain in the capacitors for some time after the power supply has been turned off. Curve A in FIG. 3 represents how output voltage Vout at output terminal 60 changes with respect to time. When power supply Vcc is turned off, output voltage Vout decreases gradually, due to the residual charge remaining in negative feedback capacitor C53. As a result, the voltage on ripple line 61 is decreased slowly with a rate corresponding to the ripple filter time constant. When the power supply is turned on before the capacitors are substantially discharged, current source circuit 55 is turned on by the voltage on ripple line 61, and transistors Q8 and Q9, transistor Q12, and the emitter follower circuit immediately become active. However, if transistors Q8 and Q9 become active before output voltage Vout of output terminal 60 is sufficiently lowered, since the voltage at input terminal 56 is immediately decreased upon the power supply being turned off, the base biases of transistors Q8 and Q9 become unbalanced. As a result, output voltage Vout of output terminal 60 drops abruptly to the ground level (point X in FIG. 3). This sudden change in the output voltage causes the loudspeaker connected to output terminal 60 to generate pop noise.